1. Field of the Invention
The present invention relates to a charging device, and more particularly, to a charging device with boundary mode control.
2. Description of the Prior Art
In the prior art, charging control of a common charging device usually includes two methods: continuous conduction mode (CCM), and discontinuous conduction mode (DCM).
FIG. 1 is a diagram of a charging device 10 with continuous conduction mode control according to the prior art. When the power switch 105 is turned on, a primary-side current Ip increases. If the primary-side current Ip increases to the peak current setting value (which is set by the peak current setting end 111 of the pulse-width modulation (PWM) controller 110), the power switch 105 is turned off. When the power switch 105 is turned off, a secondary-side current Is starts to decrease (please refer to FIG. 2). When the secondary-side current Is does not decrease to zero, the power switch 105 is turned on again. The charging device 10 includes the following characteristics and advantages: (1) Short charging time and high charging efficiency. (2) A smaller maximum value of the primary-side current Ip and a higher switching frequency, so it can be applied to a small-scale transformer to save the volume of the power converter system. (3) With the higher switching frequency, a smaller input capacitor Cin can cause small average charging current ripples. However, the charging device 10 still includes the following disadvantages, for example: (1) A detecting resistor 120 connected to the secondary-side winding of the transformer 101 is necessary for detecting the second-side current, so the PWM control chip 100 must have a pin 125 to measure the voltage across the detecting resistor 120. (2) There is a swing current phenomenon in the primary-side current Ip (please refer to FIG. 2), so longer swing current times of the transformer may result in a fault trigger of the current level. (3) There is also still a swing current phenomenon in the secondary-side current Is due to the transformer being non-ideal, so the turn-off time of the power switch 105 during charging is not fixed if the swing current phenomenon in the secondary-side current is too serious. That is, the average charging current in is not smooth and exhibits a shaking phenomenon.
FIG. 3 is a diagram of a charging device 30 with discontinuous conduction mode (DCM) control according to the prior art. When the power switch 305 is turned off, the secondary-side current Is gradually decreases to zero (please refer to FIG. 4). After a period of time, the power switch will be turned on again. The charging device 30 includes the following characteristics: (1) A detecting resistor connected to the secondary-side winding of the transformer 301 for detecting the second-side current is not necessary, and thus the PWM control chip 300 needn't have a detecting pin. (2) A swing current phenomenon does not happen in the primary-side current Ip. (3) Due to the secondary-side current Is not being detected, a swing current phenomenon won't happen in the secondary-side current Is, so the average charging current in is smoother and shows no shaking phenomenon. The charging device 30 still includes the following disadvantages, however: (1) After the power switch 305 is turned off, the secondary-side current Is decreases to zero, and when the drain voltage Vsw of the power switch 305 decreases to a reference voltage Vref due to resonance, the power switch 305 will be turned on again. At this time, the portion of turn-off time Toff of the power switch 305 occupied by the blank time Tb increases as the voltage Vsw becomes higher (shown in FIG. 4). Because the charging device 30 doesn't transmit the energy from the input voltage Vin to the output capacitor Co during the blank time Tb, the total charging time is longer and charging efficiency is lower. (2) The turn-off time Toff of the power switch gets longer and the switching frequency is smaller due to the blank time, so a larger transformer is needed. (3) Because the switching frequency is smaller, a larger input capacitor Cin is needed to cause small average charging current Iin ripples. (4) With the increase of output voltage Vout during charging, the average charging current Iin cannot be fixed and decreases gradually.
In view of this, a charging device with boundary mode control is disclosed in the present invention to improve the problems of the prior art.